13

This number is a prime.

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The only prime subscript of a triangular number that is also a square pyramidal number : 1 + 2 + 3 + ... + 11 + 12 + 13 = 1² + 2² + 3² + 4² + 5² + 6². [Beedassy]

Submitted: 2008-10-18 22:11:48; Last Modified: 2008-10-18 23:38:01.